Mammalian blood cells provide for an extraordinarily diverse range of activities. The blood cells are divided into several lineages, including lymphoid, myeloid and erythroid. The lymphoid lineage, comprising B-cells and T-cells, provides for the production of antibodies, regulation of the cellular immune system, detection of foreign agents in the blood, detection of cells foreign to the host, and the like. The myeloid lineage, which includes monocytes, granulocytes, megakaryocytes as well as other cells, monitors for the presence of foreign bodies in the blood stream, provides protection against neoplastic cells, scavenges foreign materials in the blood stream, produces platelets, and the like. The erythroid lineage provides the red blood cells, which act as oxygen carriers.
Hematopoietic stem cells (HSC) represent a rare population of bone marrow cells with the capacity to reconstitute the entire hematopoietic system. See Jordan and Lemischka, 1990; Osawa et al., 1996. Present studies aimed at understanding the basic biology of these cells are undermined by an inability to obtain purified hematopoietic stem cells. Current purification of hematopoietic stem cells utilizes multicolor fluorescence activated cell sorting (FACS) to isolate sub-populations of bone marrow cells expressing a specific pattern of cell surface markers. In addition, hematopoietic stem cells possess unique dye efflux properties which create a distinct staining profile when stained with the vital dye Hoechst 33342. These weakly Hoechst stained cells, termed side-population (SP) cells, have been found to be greatly enriched for hematopoietic activity.
There is a strong interest to obtain purified hematopoietic stem cells. Having possession of such purified stem cell will allow for identification of growth factors associated with its self regeneration. The availability of purified hematopoietic stem cells would be extremely useful in bone marrow transplantation, as well as transplantation of other organs in association with the transplantation of bone marrow. Bone marrow transplantation alone has broad clinical implications for treatment for hematological malignancies, nonmalignant blood cell disorders, autoimmune diseases, and bone marrow failures associated with cancer treatment. In addition, HSC are important targets for gene therapy, where the inserted genes promote the health of the individual into whom the purified hematopoietic stem cells are transplanted.